Role Of Neuropeptides And Biogenic Amines In Stress and Brain Inflammation

National Institute Of Mental Health

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Abstract

Our laboratory studies the central mechanisms leading to anxiety, depression, stress-related illnesses and inflammatory diseases of the brain, with the goal to develop novel therapeutic strategies for these diseases. We initially found that the hormone and brain modulator Angiotensin II (Ang II), the active principle of the Renin-Angiotensin System (RAS), is a major factor in the regulation of the stress response, and that excess Ang II AT1 receptor stimulation participates in the development of anxiety and stress-related disorders, the vulnerability to brain ischemia and cerebrovascular inflammation (6, 7, 8). For these reasons, our current focus is on the functions of brain Ang II and on the therapeutic effects of Ang II AT1 receptor blockers (ARBs). [unreadable] Ang II participates in stress by regulating the hypothalamic-pituitary-adrenal (HPA) axis activating the production and release of CRH in the paraventricular nucleus, the secretion of vasopressin, and the central and peripheral sympathetic activity (1, 2). Excess AT1 receptor stimulation translates into excessive HPA axis and sympathetic activity during stress. Our laboratory has earlier demonstrated that pretreatment with a peripheral and central ARB completely prevents the hormonal and sympathoadrenal response to isolation stress and the production of stress-induced gastric ulcers in the rat during cold-restraint, new findings with important clinical implications. We now report that the influence of ARBs during stress is not confined to hypothalamic mechanisms. We found that AT1 receptor blockade has an anxiolytic effect, and prevents the stress-induced decrease in CRH1 and benzodiazepine binding in the cerebral cortex during isolation. Thus, central inhibition of AT1 receptors counteracts the stimulation not only of the hypothalamic CRF system but of the cortical CRF system as well (5). Preservation of normal benzodiazepine binding during stress can be interpreted as protection of the cortical GABA-A system leading to decreased anxiety during stress. [unreadable] We are continuing our studies on the effects of life-long ARB administration on the life span of the stress-sensitive Spontaneously Hypertensive Rats (SHR). ARBs substantially prolonged the life span of SHR, protecting the brain, heart and kidney from hypertension-related ischemia and inflammation. These compounds had a life-long anti-stress effect, with decreased sympathetic and HPA axis activity during isolation, and decreased anxiety during the whole length of the treatment (72 weeks). This indicates end-organ protection, anti-stress and anti-anxiety effects of ARBs, and reveals that the animals do not adapt to the beneficial effects of these compounds. [unreadable] Studies on the role of AT2 receptors reveal that these receptors regulate both the central sympathetic and the peripheral adrenomedullary activities, through the control of transcription of tyrosine hydroxylase. These findings indicate that AT2 receptors participate in the regulation of the central sympathetic and adrenomedullary response to stress. In AT2 -/- mice, the brain AT1 receptors are upregulated, suggesting a mutual influence of AT2 and AT1 receptor activation in AT1 and AT2 receptor expression.[unreadable] We initiated studies to determine the effect of ARBs on inflammatory stress. Administration of the bacterial endothoxin lipopolysaccharide (LPS) produces a characteristic stress response, with stimulation of aldosterone synthesis and release from the adrenal gland, and excess expression of AT1 receptors in the paraventricular nucleus of the hypothalamus. ARBs prevent the aldosterone response to LPS, without significantly affecting the HPA axis activity, because the release of ACTH and glucocorticoids is unaffected. These results demonstrate that while ARBs have general anti-stress effects, the regulation of the HPA axis depends on the type of stress. In the case of inflammatory stress, ARBs do not prevent the HPA axis response to inflammation. Protection of the anti-inflammatory corticoid response is a beneficial effect. Our preclinical experiments in rodents suggested the possibility to test the effects of ARBs in humans. We have recently proposed the first clinical protocol to evaluate the effects of AT1 receptor antagonists in the fear-startle response in human volunteers, and the protocol will start momentarily. The goal is to determine if AT1 receptor antagonists are effective in reducing anxiety and stress in humans. [unreadable] We have earlier discovered, using SHRs, that ARBs reverse the chronic cerebrovascular inflammation characteristic of hypertension, leading to reversal of cerebrovascular remodeling and vulnerability to brain ischemia, effects dependent on the regulation of the cerebrovascular RAS and not of the circulating, hormonal RAS (11) , and unrelated to the effects of ARBs on blood pressure. These findings indicated that ARBs may also prevent or reverse inflammatory conditions of the brain unrelated to hypertension. We chose a model of acute inflammation, the administration of lipopolysaccharide (LPS). We found that AT1 receptor blockade prevented the complete peripheral and brain inflammatory response to LPS when injected in vivo to rats. We also found a significant reduction of the LPS response in cultured undifferentiated human monocytes. These findings demonstrate a role of Angiotensin II in the innate immune response, and reveal that AT1 receptor antagonists are effective anti-inflammatory compounds. Because circulating human monocytes do not express AT1 receptors, our finding indicate AT1 receptor-independent effects of ARBs. Understanding these non-Ang II effects may provide additional information for the development of novel anti-inflammatory compounds.[unreadable] Additional experiments demonstrate that there is a complete Renin-Angiotensin System in adipose tissue. Treatment with AT1 antagonists improves insulin sensitivity and increases the levels of adiponectin, a hormone released by adipose tissue and exerting anti-inflammatory effects in the vasculature (12). Our findings that ARBs stimulate the function of the peroxisome proliferator-activated receptor gamma (PPAR-gamma) reveal additional non-Ang II effects of these compounds and are important to clarify the molecular mechanisms of the anti-diabetic properties of ARBs. [unreadable] Activation of AT2 receptors has been proposed to balance AT1 receptor stimulation and to exert anti-inflammatory effects. Our findings that estrogens dramatically up regulate AT2 receptor expression indicate a possible participation of AT2 receptor activation on the anti-inflammatory effects of estrogen.[unreadable] Our results indicate that ARBs may be considered as a novel class of multitasking anti-stress, anti-anxiety, anti-inflammatory medications in the treatment of brain disorders. Because these compounds, widely used to treat high blood pressure in humans, are safe and are devoid of addictive properties, development of new compounds of this class may result in medications of great therapeutic potential. The role of AT2 receptors remains an open question.[unreadable] We continue our experiments to clarify: 1) the central regulation of the stress response with emphasis on hypothalamic and supra-hypothalamic mechanisms and the role of Ang II, the factors leading to enhanced vulnerability to stress, and the mechanisms and limitations of the anti-stress effects of ARBs and AT2 related compounds; and 2) the role and mechanism of the pro-inflammatory effect of Ang II in the brain and the mechanisms of Ang II related and unrelated anti-inflammatory effects of ARBs, and the role of AT2 receptors in brain inflammation.